This invention relates to a photocathode with a high photoelectric conversion ratio and a large area.
A photocathode is a transducer of an important kind for converting light into an electric signal. Although many kinds of photocathodes have been used in the past, there has been none having a high photoelectric conversion ratio over a wide wavelength range of incident light. A photocathode composed mainly of silver oxide, for example, has a peak in its photoelectric conversion ratio at wavelength of about 6000 .ANG. but this ratio becomes about 1/2 of the peak value at wavelength of about 4000 .ANG.. As another example, a photocathode composed mainly of a silverbismuth alloy has a peak in its photoelectric conversion ratio at wavelength of about 4500 .ANG. but this ratio drops to about 1/2 of the peak value at wavelength of about 6000 .ANG.. In order to eliminate this problem, photocathodes composed mainly of GaAs have been considered, but such photocathodes have the disadvantages of being expensive and that it is difficult to provide a large area. Moreover, since use is made of arsenic which is a harmful material, there is an additional problem of public harm in their production processes.